seteuid

SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
int seteuid(uid_t euid);
int setegid(gid_t egid);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
seteuid(), setegid():
_BSD_SOURCE || _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
DESCRIPTION
seteuid() sets the effective user ID of the calling process. Unprivi-
leged user processes may only set the effective user ID to the real
user ID, the effective user ID or the saved set-user-ID.
Precisely the same holds for setegid() with "group" instead of "user".
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
ERRORS
EPERM The calling process is not privileged (Linux: does not have the
CAP_SETUID capability in the case of seteuid(), or the CAP_SET-
GID capability in the case of setegid()) and euid (respectively,
egid) is not the real user (group) ID, the effective user
(group) ID, or the saved set-user-ID (saved set-group-ID).
CONFORMING TO
4.3BSD, POSIX.1-2001.
NOTES
Setting the effective user (group) ID to the saved set-user-ID (saved
set-group-ID) is possible since Linux 1.1.37 (1.1.38). On an arbitrary
system one should check _POSIX_SAVED_IDS.
Under libc4, libc5 and glibc 2.0 seteuid(euid) is equivalent to
setreuid(-1, euid) and hence may change the saved set-user-ID. Under
glibc 2.1 and later it is equivalent to setresuid(-1, euid, -1) and
hence does not change the saved set-user-ID. Analogous remarks hold
for setegid(), with the difference that the change in implementation
from setregid(-1, egid) to setresgid(-1, egid, -1) occurred in glibc
2.2 or 2.3 (dependeing on the hardware architecture).
According to POSIX.1, seteuid() (setegid()) need not permit euid (egid)
to be the same value as the current effective user (group) ID, and some
implementations do not permit this.
SEE ALSO
geteuid(2), setresuid(2), setreuid(2), setuid(2), capabilities(7), cre-
dentials(7)